In a power system, relays are devices that monitor the flow of electrical quantities such as current, voltage, frequency, or impedance. This guide explains the main categories—from basic electromechanical relays to modern solid-state and protective types—so you can choose the right relay for your project. Engineering use: Relays are used on feeders, transformers, buses, motors, generators, and transmission lines to protect equipment and improve system. A relay is an electrically operated switch that uses a small electrical signal to control the opening or closing of one or more sets of contacts in another circuit.